Mineral nitrogen form preference, partitioning and root morphological traits of Douglas fir and European beech under intra—and interspecific competition in-situ

Abstract Climate change challenges forestry in temperate regions, with drought and insect infestations affecting major tree species. Introducing drought-tolerant non-native species into managed forests is a widely discussed adaptive strategy. However, the impact of such interventions on forest nitrogen (N) cycling are still unknown. Here, we investigate the influence of species identity and competition on sapling N form preferences and root morphological traits of non-native Douglas fir ( Pseudotsuga menziesii ( Mirbel ) Franco ) and native European beech ( Fagus sylvatica L.). We conducted an in-situ experiment in a close-to-nature beech forest in Germany, growing beech and Douglas fir saplings for two years in bottomless pots under intra- and interspecific competition. Using highly 15 N-enriched ammonium nitrate, we tracked N uptake in plant compartments to compare the species’ inorganic N uptake form preference and allocation. Under intraspecific competition, Douglas fir and beech showed higher uptake of ammonium than nitrate. Under interspecific competition, N form uptake was complementary, with beech exhibiting a preference for nitrate. Fine root morphology did not significantly affect N form preference. Roots displayed species-specific traits; Douglas fir’s traits point to a slow resource acquisition strategy (e.g., large fine root diameter) and traits of beech to a fast strategy (e.g., high branching frequency), yet fine root morphology was unaffected by species mixing. Our findings suggest that Douglas fir and European beech saplings can coexist without hampering each other’s N nutrition. The study highlights the necessity of field-based investigations to elucidate the complex interactions governing N cycling in mixed-species forests, providing insights for climate change-adapted forest management.